1. Field of the Invention
This invention relates to a high dynamic range mass spectrometer, preferably although not exclusively of the time of flight kind.
2. Description of the Prior Art
Time of flight (TOF) mass spectrometers are often used for quantitative analysis of substances. In these applications of a TOF mass spectrometer, it will be necessary to be able to accurately determine the concentration of a substance based upon a detected ion signal. In a TOF mass spectrometer, the ion signals which are to be detected are usually fast transients and can be measured by analogue to digital conversion using a transient recorder or by ion counting as a function of time using a time to digital converter (TDC). Use of a TDC is generally preferred because it can be more difficult to obtain accurate quantitative results using a transient recorder. The use of ion counting is further preferred in an orthogonal acceleration TOF because the signals to be measured tend to be small and the ion count rates are low. Ion counting using a TDC involves the TDC detection the presence of a signal at the detector in excess of predetermined threshold. If the signal detected is in excess of a predetermined threshold then this is deemed to be indicative of the presence of an ion at the detector and the TDC, after detection of the above threshold signal, increments a counter to count the ions.
However, a problem arises with a time to digital converter when this is used to count ions in intense ion beams because most TDC's can only detect on event in a finite small time window. This means that where a TDC is used, it is not normally possible to distinguish between a single ion being detected and a multiplicity of ions being detected at the same time. This arises because a TDC cannot distinguish between different magnitudes of signal, only whether the detected signal exceeds the predetermined threshold. Accordingly, a counter connected to the TDC will only be incremented once upon detection of an above threshold signal regardless of its magnitude and therefore in the case of intense ion beams an accurate quantitative measurement cannot be made. This means that mass spectrometers incorporating such ion counters usually require there to be less than or equal to one ion per signal pulse of any substance to measured. It also means that for a single TDC there will be a relatively low dynamic range.
Attempts have been made to provide a mass spectrometer which uses one or more TDC's to count ions and in which the dynamic range can be extended for better quantitative measurements.
Thus for example, U.S. Pat. No. 5,777,326 discloses a TOF mass spectrometer in which the incoming ion beam is spread so as to be capable of being detected by three or more detectors. The signal at each detector is detected by a respective TDC and the signal from each TDC is subsequently added together. However, the problem with this type of arrangement is that simply spreading the beam over a number of detectors does not affect the intensity of the beam to a sufficient extent to significantly enhance dynamic range without a very large number of TDC's.